The present invention relates generally to solvent extraction processes for recovery of metal values from aqueous solutions and, more particularly, to the enhancement of copper/iron selectivity in the use of certain extraction reagents.
The starting material for large scale solvent extraction processing of copper is an aqueous leach solution obtained from a body of ore which contains a mixture of metals in addition to copper. The leaching medium dissolves salts of copper and other metals as it trickles through the ore, to provide an aqueous solution of the mixture of metal values. The metal values are usually leached with sulfuric acid medium, providing an acidic aqueous solution, but can also be leached by ammonia to provide a basic aqueous solution.
The aqueous solution is mixed in tanks with an extraction reagent which is dissolved in an organic solvent, e.g., a kerosene. The reagent includes an extractant chemical which selectively forms metal-extractant complex with the copper ions in preference to ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process.
The outlet of the mixer continously feeds to a large settling tank, where the organic solvent (organic phase), now containing the copper-extractant complex in solution, is separated from the depleted aqueous solution (aqueous phase). This part of the process is called phase separation. Usually, the process of extraction is repeated through two or more mixer/settler stages, in order to more completely extract the desired metal.
Where two or more mixer-settler stages are employed for extraction, countercurrent flow of the feedstock aqueous solution and the organic phase or reagent solution is employed. In a typical 3-stage extraction system, for example, the feedstock will flow through an initial mixer-settler stage ("E-1"), subsequently through a second stage ("E-2"), and then through a final mixer-settler stage ("E-3"). The organic phase will, in turn, initially contact the feedstock in E-3, encounter a subsequent contact in E-2 and a final contact in E-1. As a result, by the time the feedstock reaches mixer-settler stage E-3, substantial amounts of copper will have been extracted from it and it will be cantacting an organic phase replete in copper. Correlatively, when the organic phase reaches mixer-settler stage E-1, much of the extractant will be in the form of copper-extractant complex and the organic phase will be contacting the feedstock solution when it is in a condition wherein little, if any, of the dissolved copper has been extracted.
After extraction, the depleted aqueous feedstock (raffinate) is either discharged or recirculated to the ore body for further leaching. The loaded organic phase containing the dissolved copper-extractant complex is fed to another set of mixer tanks, where it is mixed with an aqueous strip solution of concentrated sulfuric acid. The highly acid strip solution breaks apart the copper-extractant complex and permits the purified and concentrated copper to pass to the strip aqueous phase. As in the extraction process decribed above, the mixture is fed to another settler tank for phase separation. This process of breaking the copper-extractant complex is called the stripping stage, and the stripping operation is repeated through two or more mixer-settler stages to more completely strip the copper from the organic phase.
From the stripping settler tank, the regenerated stripped organic phase is recycled to the extraction mixers to begin extraction again, and the strip aqueous phase is customarily fed to an electrowinning tankhouse, where the copper metal values are deposited on plates by a process of electrodeposition. After electrowinning the copper values from the aqueous solution, the solution, known as spent electrolyte, is returned to the stripping mixers to begin stripping again.
Among the more problematic copper bearing feedstocks treated in conventional solvent extraction processes are those which include substantial quantities of dissolved iron values. Frequently the extractant chemical employed will form an iron-extractant complex which, in turn, results in the presence of iron in the strip aqueous phase. Where electrowinning is employed to recover copper from the strip aqueous solution, the presence of iron will complicate recovery by decreasing current efficiency, by corrosively affecting cathodes, and the like. To avoid such problems, a more or less constant "bleed" of the tankhouse solution is established, with the solution bled off being circulated back into the initial feedstock or to the leech pile itself. Because such tankhouse bleed solutions contain appreciable amounts of copper and acid, efficiency of the entire system can be compromised.
The currently more favored reagents employed in recovery of copper values from aqueous solutions having iron values present are those which exhibit a relatively high degree of copper/iron selectivity, i.e., those which, under standard operating conditions, extract a high proportion of the copper present in the feedstock but only a minor proportion of the iron present. Among the reagents credited with displaying good copper/iron selectivity characteristics are those including hydroxy aryl oxime extractants such as long chain alkyl or alkenyl solubilized hydroxy aryl aldoximes and ketone oximes. See, for example, Birch, "The Evaluation of the New Copper Extractant `P-1`" appearing in the Proceedings of the 1974 International Solvent Extraction Conference, pp. 2837-2871, wherein "high selectivity against Fe (III) . . . in the sulphate system" is attributed to a reagent containing a 2-hydroxy-5-nonyl benzaldoxime extractant.
None of the above-noted hydroxy aryl oxime-containing reagents has proven to selective of copper in copper and iron-bearing solutions to the complete exclusion of iron. As a result, recovery of copper from such solutions necessitates at least some bleeding off of tankhouse solutions with losses to the overall economy of the system.
Of interest to the background of the present invention are certain prevailing practices in the art of constructing and operating continuous, multi-stage solvent extraction plants. Despite significant variations among different commercial plants in terms of reagent used, organic and aqueous flow rates, length to width ratios of mixer-settlers, and the like, all plants are uniform in their practice of providing substantially identical residence times for feedstock/reagent mixtures in each of the two or more mixer-settler stages of the plant. This uniformity of residence time is ordinarily accomplished by using mixer-settler tanks of equal volume in all extraction stages and by providing for uniform admixture of the countercurrently flowing aqueous feedstock and organic phase at each extraction stage (e.g., through use of identically-sized impellers rotating at the same rate). Indeed, once the number of stages and the phase ratio for a given system have been decided, the residence time (and hence the mixer-settler size) is determined with no apparent consideration given to the possibility that there could or should be any variation between extraction stages. See, e.g., Godfrey, et al. Chemistry and Industry, No. 17, pp. 713-718 (1977). This is due in part to the observation that the nearly completely "loaded" organic phase will gain little (considered in terms of additional copper extracted) by an extended contact with the aqueous feedstock in, e.g., an E-1 mixer-settler stage, and correlatively, the copper-depleted feedstock will not yield significant additional amounts of copper by reason of extended contact with the organic phase in, e.g., the E-3 stage. The uniform extraction mixer-settler stage residence times are generally set in the range of from about 2 1/2 minutes to about 3 minutes. See, e.g., Tumily, et al., Adv. In Extractive Metallury Int'l. Symp. 3rd., pp. 123-131 (1977) (London).